This invention relates to an adaptive braking system for a fluid pressure operated vehicle braking system, and, more particularly, to a failure detection circuit for such an adaptive braking system which senses a malfunction of the electrically actuated solenoid valve and blows a fuse when such a malfunction occurs. However, if the malfunction is due solely to a decrease in the voltage produced by the vehicle electrical supply to below a predetermined level, the fuse is not blown and the adaptive braking control circuit is disabled temporarily until the low voltage condition no longer exists.
Governmental regulations require, as a practical matter, that all air braked vehicles be equipped with an adaptive braking control system. However, these governmental regulations also require that, in case of a malfunction of the control system, that the normal effectiveness of the vehicle fluid pressure braking system not be deprecated, and that the control system not exert an inordinate drain on the vehicle electrical supply system. Since there are potential failures, such as a shorted solenoid valve, which might result in an inordinate drain of the vehicle electrical supply, and since there are other failure modes, such as the solenoid valve being actuated continuously, which may result in failure or reduced effectiveness of the normal vehicle fluid pressure braking system, adaptive braking systems, particularly those used on semi-trailers, must be protected by a fuse which is blown if one of the aforementioned malfunctions occurs. However, there are other types of malfunctions in which it may appear that the solenoid is malfunctioning, but the malfunction of the solenoid is due only to a reduced voltage output from the vehicle electrical power supply. This decreased voltage may be a purely transitory phenomena, as might occur when the vehicle engine is cranked for a sufficiently long time with a weak battery while the vehicle operator effects a brake application. Since the vehicle stoplight switch, which is actuated by a brake application, enables the vehicle adaptive braking system, and since during cranking of the engine, a self-test function is performed on the adaptive braking system, it is possible for the low voltage condition caused by an old battery and sufficiently long engine cranking to cause a failed condition of the solenoid. Since this failed condition is due to lack of sufficient electrical power, and since this may be a purely transitory phenomena, it is undesirable to blow a fuse under these circumstances, although it is desirable that the adaptive braking system be disabled until the low voltage condition no longer exists.